The primary objective of this study is to describe in patients with plaque psoriasis the dose level at which no dose limiting toxi cities occur following a single intravenous infusion of MEDI-507. Secondary objectives: 1) to evaluate serum concentrations of MEDI-507, 2) to evaluate the pharmacodynamic effects of MEDI-507 on the absolute lymphocyte count and the dynamics of the following lymphocyte phenotypes: CD2, CD3, CD19(+)20(+) and CD16(+)56(+) in the peripheral blood, 3) to desribe CD2 receptor occupancy by MEDI-507, 4) to describe CD2 receptor occupancy by MEDI-507, 5) to describe the change in the clinical grade and histopathology of plaque psoriasis in MEDI-507 recipients, 6) to evaluate development of anti-MEDI-507 antibodies. T-cells are an essential part of the pathophysiology of psoriasis, and agents active against T-cells have ameliorated the disease. MEDI-507, a humanized monoclonal antibody, binds to the CD2 receptor found on the surfacces of T-cells and natural killer (NK) cells and may suppress the function of these cells or eliminate them from the circulation. This open-label protocol is the first administration of MEDI-507 to psoriatic patients. It will describe safety, adverse reactions, lymphocyte population kinetics, change in psoriasis clinical grade and histopathology, specific serum cytokine concentrations, serum MEDI-507 concentrations and MEDI-507 immunogenicity.